Testing of bimetallic actuators with radio frequency induction heating

ABSTRACT

A method and apparatus for testing a thermal tripping mechanism having a bimetallic element deflectable in response to overload current by heating the bimetallic element by radio frequency induction heat.

FIELD OF THE INVENTION

The present invention relates generally to testing ofbimetallic-motivated mechanical and electro-mechanical control circuits,circuit breakers, switches and similar devices, and more particularly toa method and an apparatus to quickly, effectively, and at a low costthermo-mechanically test bimetallic-motivated electromechanical circuitbreakers by heating the bimetallic actuator of the circuit breaker.

BACKGROUND OF THE INVENTION

In electrical control circuits and in electrical circuit breakers,thermal responsive controls or actuators may be connected in the linesor in the circuit breaker to open the circuit or circuit breaker in theevent of abnormal current conditions and the like. Such thermal overloadcontrols may take a variety of different forms including bimetalelements forming a current conducting contact of a switch or circuitbreaker. If the current rises above a selected level, the heating effecton the bimetal element, with well known differences of thermal expansionand contraction of the individual elements, is such as to actuate theswitch or the thermal tripping mechanism of the circuit breaker.Electro-mechanical circuit breakers open the electrical current pathwhen the electrical circuits that they protect are conducting excessivecurrent. The duration of opening time, under overload currentconditions, is controlled by a bimetallic actuator.

The thermal tripping function of a circuit breaker is dependent onthermal heating which is created during operating conditions ifexcessive current flows through the circuit breaker. An example of suchthermal trip mechanisms is described in U.S. Pat. No. 5,614,878 assignedto the assignee of the present invention and is hereby incorporated byreference. The heating causes the bimetal to deflect which in turncauses a mechanical force that will trip the circuit breaker,terminating the excessive current flow. Each circuit breaker must betested to assure that the tripping times conform with the trip timelimits specified in Circuit Breaker Performance Specifications. It isnecessary to test the movement of the bimetal element because thedeflection versus temperature characteristic within the operating rangeof the bimetal is very flat. This test is called thermal calibration.

Subjecting the entire circuit breaker to heat in order to test theresponse of the bimetal mechanism to temperature is impractical becauseof the presence of other temperature sensitive subcomponents susceptibleto overheating, i.e. plastic parts, lubricants, etc., which could leadto the destruction of the circuit breaker. Similarly, direct contactapplication of heat is too limiting because access to the inner portionsof the circuit breaker is oftentimes physically impracticable and alwaysextraordinarily time consuming.

One typical method of thermally calibrating circuit breakers is byinjecting a fixed level of electrical current (i) for a given timeduration (t). This controlled unit of electrical energy (i²t) causes abimetal deflection, which functionally converts the electrical energy tomechanical energy manifested as a force operating through a distance.The mechanical energy so produced by the bimetal is used to open thecircuit breaker's trip release actuator.

The prior art methods of electrical current injection for thermalcalibration uses trial and error to identify circuit breakers that havethermal calibration abnormalities. Several failed attempts at suchelectrical current injection testing must be completed before thebreaker can be identified as one that cannot pass the thermalcalibration.

The thermal calibration test requires that each circuit breaker bemechanically and electrically connected to a test apparatus and amechanical test fixture. This requires a human operator to insert andremove the circuit breaker from the test apparatus and fixture. Theperformance of the actual test, as contrasted to its setup andconnection, and to cool the circuit breaker to ambient for retestingtakes from thirteen (13) to twenty-six (26) seconds to perform. Repeattesting attempts that are presently performed on circuit breakers thathave characteristics that are out of acceptable limits includes resetuptime and reconnect time in addition to the actual test time. When setuptimes, connection times, operator action times and repeated cool toambient times (approximately two (2) hours between each test) for theretest are summed, the amount of time needed to perform the thermalcalibration cycle of tests before a circuit breaker can be identified asone that cannot be thermally calibrated can reach five (5) hours. As istypical in the prior art after three (3) bad tests for a circuitbreaker, it is sent for teardown.

Each circuit breaker's electrical to mechanical energy conversion ratiovaries slightly from a statistical average or norm. The energyconversion differences between one circuit breaker and a secondidentical circuit breaker can have many causes. Most are caused by minorelectrical resistance changes in the circuit breaker's electricalcurrent path. These variations are small, occur infrequently, and rarelyproduce a thermal calibration failure of the circuit breaker.

Each circuit breaker's trip release actuator also has a unique releaseenergy (force×distance) requirement. The relative differences in theunique trip release actuator energy requirement between one circuitbreaker and another identical circuit breaker can be large. The energyrequired to actuate the trip release actuator of such circuit breakerswhich sustain a thermal calibration failure is excessive. Anunacceptably high level of new assembled circuit breakers can oftentimessustain a thermal calibration failure.

If a circuit breaker trips during testing but either before a minimumset time or after a maximum set time, an adjustment must be performedand typically involves manually adjusting a screw which will bring thecircuit breaker trip mechanism into acceptable operating limits. Thecalibration adjustment involves changing the position of the appliedforce, created by the thermal expansion of the bimetal, to that newposition which initiates the release of the trip mechanism within therequired time. Calibration is performed, for most thermostatic bimetaloperated circuit breakers, by such a screw adjustment. Although such acalibration adjustment might appear by the simplicity of its design tobe insubstantial, it is a significant step in the manufacture of thecircuit breaker without which a failed circuit breaker could notsubsequently pass calibration retests and would be discarded.Furthermore, the calibration change adjustment requires an enormousamount of time involving an elapsed test time, the time to perform theadjustment, the time for the circuit breaker to cool down, followed byat least one additional elapsed test time duration. Frequently, duringcircuit breaker calibration, the above cycle of steps is oftentimesrepeated many times thereby driving production costs even higher.

Thus, the thermal calibration test using the electrical currentinjection method must be repeated after each manual screw adjustment ismade to determine if the circuit breaker has been brought intoacceptable operating limits. Repeated electrical current injectionthermal calibration testing of an as yet unknown mechanically failedelectric circuit breaker is obviously a total waste of time ofproduction assets and simply increases costs. The ability to reliablydetermine and isolate, mechanically failed circuit breakers prior toperforming electrical testing of the electromechanical components of thecircuit breaker, enhances effective failure analysis, reduces productiontime testing, and increases cost savings.

To further exacerbate the situation, when a circuit breaker's releaseactuator requires more mechanical operating energy than that which canbe provided within a required time limit by the circuit breaker'sbimetal, the circuit breaker cannot be calibrated. Although manyconditions can cause the trip release actuator to require such excessiveenergy, typically they include part s or subcomponents that are out oftolerance, improper lubrication, excessive friction, damaged parts orsubcomponents, improper assembly, and unknown or subtle undetectablemechanical factors. Prior art electric current injection calibrationprocesses will not identify those breakers which have these subtlemechanical conditions or anomalies. Accordingly, several failed thermalcalibration tests performed in accordance with the prior art electricalcurrent injection calibration processes oftentimes must be completedbefore it can be concluded that the circuit breaker is not capable ofbeing successfully thermally calibrated. This further increasesproduction costs.

Thus, the determination that any particular circuit breaker which hasfailed a thermal calibration test is inevitably incapable of beingsuccessfully calibrated, presently requires excessive amounts oftesting, incurs substantial costs, and involves inordinate amounts oftime without any positive benefit.

Thus, if confirmation of the proper functioning/performance of all themechanical components of the circuit breaker can be achieved early inthe production and qualification of the circuit breaker, then theelectrical injection test which is used to test and qualify theelectrical response of the components of the circuit breaker can besubsequently performed on mechanically confirmed qualified circuitbreakers. Any failure to conform to electrical test standards during asubsequent electrical injection test would therefore be indicative of anelectromechanical problem to the exclusion of a mechanical problem. Theprior art devices and methods to qualify the circuit breaker employ theelectrical current injection test to simultaneously perform both thequalification test for the mechanical performance as well as theelectromechanical performance of the circuit breaker. Accordingly, theprior art testing methods using current injection are incapable ofdistinguishing between a mechanical performance problem and anelectromechanical performance problem in a failed circuit breaker.

Accordingly, there exists a substantial need for a process which canachieve rapid, effective, and low cost identification of those circuitbreakers that are not capable of being thermally calibrated.

Accordingly, there exists a need for a process which avoids theelectrical current injecting test to thermally test a circuit breakerbimetallic mechanism.

Accordingly, there exists a need for a process which can test themechanical performance of a circuit breaker independent of and beforeany electrical energy test of the mechanical and electrical componentsof the circuit breaker.

It would thus be an advantage over the prior art to provide a methodwhich will be able to confirm the mechanical performance of the circuitbreaker, independent of and prior to conducting any electricalperformance test, and to do so without the need for any test orinterface hookups.

Accordingly, it would be a further advantage over the prior art toprovide a method to increase the production through-put in themanufacture of electrical circuit breakers by qualifying the thermal andmechanical performance of the circuit breaker early in the production ofthe device.

SUMMARY OF THE INVENTION

The present invention relates to a method and an apparatus for testingthe thermal trip setting of a thermal tripping mechanism utilized in anelectric circuit breaker, wherein the thermal tripping mechanismincludes a bimetallic element deflectable in response to overloadcurrent through the circuit breaker by heating the bimetallic element byradio frequency induction heat to induce a thermal trip of the thermaltripping mechanism.

In another embodiment, a method and an apparatus is provided for testingthe thermal trip setting of a thermal tripping mechanism utilized in anelectric circuit breaker, wherein the thermal tripping mechanismincludes a bimetallic element deflectable in response to overloadcurrent through the circuit breaker by heating the bimetallic element byradio frequency induction heat to the temperature at which it is desiredto have a thermal trip of the thermal tripping mechanism.

In a further embodiment, a method and apparatus is provided for testingthe thermal trip setting of a thermal trip mechanism having an elementfor operating a control device in an electrical switch, wherein thethermal tripping mechanism includes a bimetallic element deflectable inresponse to a current flow through the electrical switch, by heating thebimetallic element by radio frequency induction heat to the temperatureat which it is desired to have a thermal trip of the thermal trippingmechanism.

In yet a further embodiment, a method and apparatus is provided fortesting the thermal trip setting of a thermal tripping mechanism in anelectric circuit breaker, wherein the thermal tripping mechanismincludes a bimetallic element deflectable in response to overloadcurrent through the circuit breaker, by heating the bimetallic elementby radio frequency induction heat to a selected temperature whereby saidbimetallic element deflects in response to said heating.

DETAILED DESCRIPTION

With the foregoing and other objectives in view and to overcome theabove-described limitations of the prior art, there is provided, inaccordance with the present invention, a method of testing the thermaltrip setting of a thermal tripping mechanism utilized in an electriccircuit breaker, wherein the trip mechanism includes a bimetallicelement deflectable in response to overload current through the circuitbreaker by heating the bimetallic element by radio frequency inductionheat to a predetermined level to induce a thermal trip of the thermaltripping mechanism.

Radio frequency induction heating of the bimetal actuator will permit acalibration pretest without requiring a mechanical or electricalconnection to the circuit breaker, or to a test fixture. The method ofthe present invention significantly increases the efficiency of thethermal calibration process by quickly and accurately identifying thosecircuit breakers that require excessive energy to release the thermaltripping mechanism of the circuit breaker. Thus, in accordance with theprocess of the present invention, the identification and screening outof those circuit breakers that are not capable of subsequently beingcalibrated can be accomplished independent of and before anyelectromechanical testing/calibration of the circuit breaker, and isaccomplished within a period of seconds at very low cost. Moreover, afurther benefit of the process of the present invention is that iteliminates the expensive, time consuming repeated attempts at thermalcalibration that are required to be performed on circuit breakers thathave failed because of responses which are out of limits. The method ofthe present invention can be accomplished with minimal human operatorintervention and within a period of seconds, i.e. the time it takes forthe temperature of the bimetal element of the thermal tripping mechanismto be raised to the temperature that will trip the circuit breaker.Thus, if a circuit breaker has an out of limits trip release actuatorenergy requirement as determined by the process of the presentinvention, it will not trip and can be removed from any further attemptsat calibration or testing. Thus, the method of the present inventionresults in improving the speed of the qualification of circuit breakers,eliminates the scraping of falsely rejected circuit breakers or theircomponents, and results in saving the lost time expended on attempts tocalibrate circuit breakers which cannot be calibrated. In addition, themethod of the present invention results in the ability to correctlyidentify test failures as mechanical by a separate defined test. Thisenables subsequent testing for electromechanical performance to beconducted only on circuit breakers that have mechanically qualifiedbimetallic trip mechanism.

Experimental tests have been carried out with BQD, single pole, 20ampere circuit beakers. More specifically, radio frequency inductionheating was applied during several test cycles to induce the thermaltrip of the thermal tripping mechanism of the circuit breakers. Thetests were conducted with a radio frequency induction heater, operatingfrequency range of 250-750 KHz, maximum output heating power of 2 KW,work coil reactive power of 50 KVa, and input power of 3 KW maximum. Theinduction coil used in the tests are {fraction (3/16)} inch diametercopper tubing formed into 1½ loops with an outer loop diameter of 2¼inches. The induction heater was operated during the tests at 450 KHzand 80% heat control.

Induction heating is well known in the prior art and is described inmany treatises as well as in McGraw-Hill Dictionary of Scientific andTechnical Terms, 1972, “Induction Heating” pp 94-97, which is herebyincorporated by reference.

The tests show the effects of radio frequency induced heating of samplecircuit breakers on the response time of the bimetallic element tothermally trip the thermal tripping mechanism under various conditionsand positions relative to the induction coil. The results of the testingare set forth in Table 1.

TABLE 1 Samples are BQD Single Pole, 20 amp, Circuit Breakers Sample 1 23 4 TEST 1 (at room temperature) Time Heating to trip (sec) 12.0 10.58.9 11.0 TEST 2 Sample (same samples at room temperature after Test 1“Retest”) Time Heating to trip (sec) 10.7 12.4 9.8 13.6 12.4 12.0 10.213.7 12.8 12.0 10.0 13.7 10.9 11.7 8.8 13.9 12.4 12.4 10.1 13.2 12.812.6 9.0 14.4 11.9 13.3 9.7 12.5 11.2 11.1 8.3 13.1 TEST 3 (Breakerpositioned standing on its length on top of coil and test performedimmediately after a first test: “Hot Retest”) Time Heating to trip (sec)8.0 6.5 5.5 7.5 TEST 4 (Breaker positioned standing on its length on topof coil and performed after breaker was allowed to cool after first testbut not to room temperature: “Semi-Cool Retest”) Time Heating to trip(sec) 7.8 8.3 6.8 9.0 TEST 5 (Breaker positioned standing on its lengthon top of coil with load end of breaker centered in coils and re-testperformed after samples were cooled but not to room temperature “SemiCool-Retest”) Time Heating to trip (sec) 11.5 10.9 3.5* 13.0 TEST 6(Breaker positioned on its side on top of and centered in coils andre-test performed while sample was still warm “Warm Retest”) TimeHeating to trip (sec) 4.5 3.0 1.9 3.9 TEST 7 (Breaker positioned on itsother side on top of coil and re-test performed while sample was hot -“Hot-Retest”) Time Heating to trip (sec) 2.9 6.3** 2.8 6.0** *Anomolousresult. **Improperly positioned.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the true spirit and scope of the presentinvention.

What is claimed:
 1. A method for testing the thermal trip setting of acircuit breaker having a bimetallic element in a thermal trippingmechanism, the method comprising the steps of: placing the circuitbreaker over an induction coil coupled to a radio frequency inductionheater; heating the bimetallic element with induced heat from thefrequency induction coil; determining trip time of the thermal trippingmechanism; determining if the circuit breaker meets a predeterminedtripping time; selecting the circuit breaker for further testing if itmeets the predetermined tripping time; and, rejecting the circuitbreaker for further testing if it does not meet the predeterminedtripping time.
 2. The method for testing of claim 1, wherein the step ofplacing the circuit breaker over an induction coil includes positioningthe circuit breaker in one of a vertical and horizontal aspect withrespect to the induction coil.
 3. The method for testing of claim 1,wherein the step of heating the bimetallic element comprises heating toa temperature that will trip the tripping mechanism.
 4. The method fortesting of claim 1, wherein the step of heating the bimetallic elementcomprises heating to a predetermined temperature whereby the bimetallicelement deflects in response to the heating.